Steel sheets are generally covered with a metal coating, the composition of which varies as a function of the final use of the steel sheet. This coating can, for example, be zinc, aluminum, magnesium or their alloys, can include one or more layers and can be applied using different coating technologies known to a person skilled in the art, such as, for example, vacuum deposition methods, hot-dip coating or electro-deposition. In the remainder of this description, the term “metal coating” will also be used to designate a coating that includes metal as well as a coating that includes metal alloy.
The metal coating can first of all be applied by hot-dip coating, whereby this process generally comprises the following steps:                Annealing of the steel sheet as it passes through a furnace under an inert or reducing atmosphere to limit the oxidation of the surface of the sheet;        Dip-coating of the sheet as it passes through a bath of metal or metal alloy in the liquid state so that when it exits the bath, the sheet is coated with the metal/metal alloy.        After the sheet exits the liquid bath, the layer of metal/metal alloy is dried by spraying a gas on the surface to guarantee a uniform and regular thickness of this layer.        
During the annealing step, before the steel sheet enters the metal bath (in the following portion of the text the terms “metal bath” and “metal layer” are also used to designate any metal alloy bath and the corresponding metal alloy layers) the sheet is generally heated in a direct flame or radiant tube annealing furnace. However, in spite of numerous measures that are taken, such as the control of an inert atmosphere, the use of these furnaces to heat the steel sheet can lead to the formation of metal oxides on the surface, which must then be removed to ensure the proper wettability of the liquid metal on the surface of the steel sheet and to prevent the occurrence of uncoated areas on the surface of the sheet.
This problem is encountered in particular when the composition of the steel includes significant quantities of easily oxidized elements such as Si, Mn, Al, Cr, B, P etc. For example, an IF (Interstitial-Free) steel that contains 0.2% by weight Mn, 0.02% by weight Si and 5 ppm B is already subject to these problems of wettability as a result of the presence of B which rapidly diffuses to the surface of the sheet and precipitates the oxides of Mn and Si in the form of continuous films, leading to poor wetting.
More generally, the risk of poor wetting by the liquid metal is also encountered in all high-strength steels because they contain at least one of these easily oxidized elements, such as Dual Phase steels, TRIP (Transformation Induced Plasticity) steels, TWIP (TWinning-Induced Plasticity), electrical steels, etc.
For Dual Phase steels, the quantity of Mn is generally less than 3% by weight, with the addition of Cr, Si or Al in quantities generally less than 1% by weight. For TRIP steels, the quantity of Mn is generally less than 2% by weight associated with a maximum of 2% by weight of Si or Al. For TWIP steels, the quantity of Mn can be up to 25% by weight, combined with Al or Si (maximum 3% by weight).
The metal coating can also be applied by electro-deposition. In this method, the steel sheet to be coated is immersed in an electrolyte bath in which one or more soluble anodes are also immersed, the anodes include the metal or the metal alloy corresponding to the coating to be applied to the surface of the sheet. The application of an electric current to the electrolyte bath causes the dissolution of the metal or the metal alloy of which the anode or anodes are made and the ions thereby formed are deposited on the surface of the steel sheet to form a layer of metal or metal alloy coating. Prior to entering the electrolysis bath, the steel sheets must undergo a pickling step to remove the metal oxides that are present on the surface. In fact, for the electrolysis process to be effective, the medium must necessarily be a conductor, which is not the case if metal oxides are present on the surface of the steel sheet to be coated. Moreover, the presence of metal oxides can influence the germination and growth of the deposit and thus lead to problems of adherence and quality of the coating (microstructure, density etc.).
The metal coating can also be applied by vacuum deposition. The vacuum deposition techniques principally require three components:                A source, which constitutes or contains the material to be deposited. This source can be, for example, the crucible of a vacuum evaporator or a sputtering target. The material to be deposited must leave this source in the form of ions, atoms or groups of atoms or groups of molecules;        A substrate, which corresponds to the part to be coated. The material originating from the source is affixed to the substrate to form germs (nucleation), which gradually develop (growth) and result in a more or less ordered coating layer;        A medium, which separates the source from the substrate and which is the location of the phenomenon of transfer of material in the vapor phase.        
A distinction is made among different types of vacuum deposits as a function of, among other things, the means used to form the vapor phase. If the vapor phase results from a chemical reaction or the decomposition of a molecule, the process is called CVD, or chemical vapor deposition. On the other hand, if this vapor is produced by a purely physical phenomenon such as thermal evaporation or ion sputtering, the process is a physical vapor deposition or PVD. PVD deposition processes include sputtering, ion implantation and vacuum evaporation.
However, regardless of the vacuum deposition technique used, it requires a preparation of the surface so that the surface of the steel sheet to be coated is free of metal oxides to guarantee the proper adherence of the metal coating and to thereby prevent problems of delamination of the coating.
Regardless of the coating method used, the surface condition of the steel strip before coating is an important factor in the quality of the final coating. The presence of metal oxides on the surface of the steel sheet to be coated prevents the proper adherence of the coating to be applied and can result in zones in which there is no coating on the final product or problems related to the delamination of the coating. These metal oxides can be present in the form of a continuous film on the surface of the steel sheet or in the form of discontinuous points. The metal oxides can also be formed during different steps of the process and their composition varies as a function of the grade of steel of which the sheet in question is made. Oxides of this type include, for example, the iron oxides FeO, Fe2O3, aluminum oxide Al2O3 as well as MnSiOx or AlSiOx.
The removal of these metal oxides requires the execution of an additional process step, i.e. pickling. In the remainder of this description, pickling means any method that results in the removal of the metal oxides formed by oxidation of the underlying metal layer so that this metal layer appears on the surface, in comparison with, for example, a brightening method which, although it is a process that removes metal oxides, is intended only to remove the surface layer of metal oxides without exposing the underlying metal layer.
This removal of metal oxides can be accomplished, for example, by vacuum pickling by magnetron pulverization, which is also called etching. This process includes creating a plasma between the strip and an auxiliary electrode in a gas that makes it possible to generate radicals and/or ions. Under normal operating conditions, these ions are accelerated toward the surface of the strip to be pickled and blast away surface atoms, thereby eliminating the metal oxides present on the surface. This method depends to a great extent on the thickness of the layer of metal oxides to be removed and, depending on the composition of these metal oxides, can generate electric arcs. The process is therefore unstable and not very robust. In addition, it sets a severe limitation on the speed of the line to obtain a good result, which poses productivity problems.
It is also possible to pickle the strip by passing it through one or more successive baths of strong acids such as hydrochloric acid or sulfuric acid, selected as a function of the nature of the metal oxides on the surface and held at a temperature of approximately 80-90° C. This process generates large quantities of effluents which require subsequent treatment and is not environmentally friendly.
In addition, this type of pickling poses the problem of controlling the thickness of metal oxides removed to guarantee the proper adherence of the subsequent coating.
Finally, it is possible to remove all or part of the layers of metal oxides by mechanical action, for example by using a shot-blasting process in which the metal oxides are removed, for example, as a result of the multiple impacts of small abrasive particles projected with sufficient kinetic energy. However, this type of process directly impacts the surface of the strip and is also complicated to implement. Moreover, these processes require working in specific conditions, such as an inert or reducing atmosphere, for example, to prevent the re-oxidation of the metal surfaces by contact with air.